Fix Git Installation to satisfy Gitlab's requirements

With this PR:
 * we use a newer git version (1.8.3.1) from the git-core ppa repository
 * we configure the git `user.name` and `user.email`

Both are requirements of Gitlab, which can be verified using this command:
```
cd /home/git/gitlab
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
```

After this PR it should not complain anymore about the git version (>=1.7.10) and user.name and user.email configuration